A conventional cellular phone system 5 is shown in FIG. 1A. Such systems are currently in widespread use in the United States. As illustrated in FIG. 1A, system 5 has a fixed number of channel sets distributed among the base stations 12, 13 serving a plurality of cells 11, 16 arranged in a predetermined reusable pattern. Typical cell areas range from 1 to 300 square miles. The larger cells typically cover rural areas and smaller cells cover urban areas. Cell antenna sites utilizing the same channel sets are spaced by a sufficient distance to assure that co-channel interference is held to an acceptably low level.
A mobile unit 10 in a cell 11 has radio telephone transceiver equipment which communicates with similar equipment in base station sites 12, 13 as the unit moves from cell to cell. Each base station 12, 13 relays telephone signals between mobile units 10 and a mobile telecommunications switching office (MTSO) 17 by way of communication lines 18. The lines 18 between a cell site and the MTSO 17, typically T1 lines, carry separate voice grade circuits for each radio channel equipped at the cell site, and data circuits for switching and other control functions. The MTSO 17 is also connected through paths 19 to a switched telephone network 15 including fixed subscriber telephone stations as well as various telephone switching offices.
MTSO 17 in FIG. 1A includes a switching network for establishing call connections between the public switched telephone network 15 and mobile units 10 located in cell sites 11, 16, and for switching call connections from one cell site to another. In addition, the MTSO 17 includes a dual access feeder for use in switching a call connection from one cell site to another. Various handoff criteria are known in the art and utilize features such as phase ranging to indicate the distance of a mobile unit from a receiving cell site, triangulation, and received signal strength to indicate the potential desirability of a handoff. Also included in the MTSO 17 is a central processing unit for processing data received from the cell sites and supervisory signals obtained from the network 15 to control the operation of setting up and taking down call connections.
A conventional base station 12 is illustrated in FIG. 1B. A radio controller unit 22 provides the interface between the T1 lines from the MTSO and the base station radio equipment. Transmitters 23, one for each channel serviced by the base station, are driven by circuit 22, which supplies each transmitter with an analog voice signal. Next, the signals are passed to a separate nonlinear power amplifier for each channel, or the signals may be combined and applied to a single linear power amplifier 24 as shown in FIG. 1B. The output of power amplifier 24 is applied through duplexer 25 to antenna 26, to be broadcast into the cellular area serviced by the base station.
Signals received in antenna 26 are applied through duplexer 25 to filter 27. Filter 27 isolates the entire cellular band signal from adjacent bands and applies it to receivers 28, one for each channel. The analog voice signal outputs of receivers 28 are applied to circuit 22. Base station 20 may optionally include a diversity antenna 26' and corresponding diversity filter 27' and a plurality of diversity receivers 28', one for each associated main receiver 28. Where implemented, the outputs of diversity receivers 28' are applied to circuit 22, which would thus include circuitry for selecting the strongest signal as between corresponding receivers 28 and 28' using known techniques.
In densely populated urban areas, the capacity of a conventional system 5 is severely limited by the relatively small number of channels available in each cell 11, 16. Moreover, the coverage of urban cellular phone systems is limited by blockage, attenuation and shadowing of the RF signals by high rises and other structures. This can also be a problem with respect to suburban office buildings and complexes.
To increase capacity and coverage, a cell area can be subdivided and assigned frequencies reused in closer proximities at lower power levels. Subdivision can be accomplished by dividing the geographic territory of a cell, or for example by assigning cells to buildings or floors within a building. While such "microcell" systems are a viable solution to capacity and coverage problems, it can be difficult to find space at a reasonable cost to install conventional base station equipment in each microcell, especially in densely populated urban areas. Furthermore, maintaining a large number of base stations spread throughout a densely populated urban area can be time consuming and uneconomical.
AT&T has proposed a system to solve the problem of coverage in urban areas without having to deploy a large number of conventional base stations. The system is shown and described with respect to FIG. 1 of AT&T's European Patent Application No. 0 391 597, published on Oct. 10, 1990. In that system a grid of antenna sites 40 is placed throughout the microcellular system. An optical fiber network 42 interconnects the antennas with the base station 44. Optical wavelength carriers are analog modulated with RF mobile radio channels for transmission through the optical fiber network 26 to the antennas sites 22. A detector circuit 27 is provided for each antenna site 22 to receive the modulated carrier and reconstruct an RF signal to be applied to the antenna sites 22, for transmission into the microcell area 21. RF signals received at antenna sites 22 from mobile units are likewise modulated onto a fiber and transmitted back through optical fiber network 26 to base station 25. All of the channels transmitted from base station 25 are distributed to all antenna sites 22. Also, all the channels transmitted from the base station 25 can be received from the mobile units in any microcell 21 and transmitted via optical fiber to base station 25.
The above-described AT&T system has certain limitations. The ability to analog modulate and demodulate light, the limitations imposed by line reflections, and path loss on the fiber all introduce significant distortion and errors into an analog modulated signal and therefore limit the dynamic range of the signals which can be effectively carried via an analog system, especially in the uplink direction. These factors limit the distance from the base station to the antenna sites.
Moreover, in AM systems an out-of-band signal is required to transmit control and alarm information to and from the antenna sites, again adding to the expense of the modulation and demodulation equipment. Moreover, provision of other services such as paging systems, personal communications networks (PCN's) or mobile data services are not easily added to analog AM systems such as that shown in AT&T's European application.
Furthermore, the AT&T system teaches the use of dedicated fiber lines installed for each remote antenna site. It would be desirable if preexisting transmission lines or fiber paths could be utilized so that installation of new fibers could be avoided.
Another approach to increasing coverage is disclosed in U.S. Pat. No. 4,932,049 to Lee. The Lee patent describes a "passive handoff" system wherein a cell is subdivided into several zones, with a directional antenna oriented to cover each zone. All the antennas in the cell are serviced by the same set of transmitters and receivers. A zone switch is used to selectively connect the transmitters and receivers to the antenna units. In operation, the antenna best able to service a mobile unit on a given channel is connected to the transmitter/receiver pair assigned to the mobile unit by the MTSO, and the other antennas are disconnected from that transmitter/receiver pair. To control the switching of transmitters and receivers to the antennas, a scanning receiver continuously polls the strength of signals received at the antenna units on all active channels in the cell. The zone having the best receiver signal strength is selected as the active zone for the associated channel. The system disclosed in the Lee patent thus allows for improving communications with mobile units while at the same time reducing interference with other cells by directionalizing and limiting overall signal strength in a cell.